This invention relates to remote devices that both provide output to and receive operating power from a utilization mechanism over a two-wire connection.
It is often desirable to monitor the pressure or temperature of a fluid or gas in an industrial process and to signal when a certain threshold has been exceeded. There are several devices known in the art that provide such monitoring. These devices may compare the measured value of pressure or temperature to a programmed threshold and provide a switch transition to indicate that the threshold has been exceeded.
Threshold detection and switching functions can be performed using a variety of electromechanical or electronic devices. Electromechanical pressure and temperature switches employ mechanical means to open or close a switch contact or set of switch contacts when certain conditions are satisfied. Such electromechanical switches generally require no power to operate, and therefore the only wire connection that need be made to the device is a two-wire connection to detect the switch state. However, electromechanical switches are in general less flexible than electronic switches in that the threshold must be set at the factory or adjusted in the field using specialized equipment.
Electronic pressure and temperature switches offer various modes of operation, adjustable thresholds and other features that can be programmed by the user in the field. They may include circuitry to monitor, display and communicate information about the overall status and health of the device and system. While these devices offer more flexibility, they typically require additional wires to supply power for the electronic circuitry. They may also require a battery or other power source at the switch. This additional wiring or local power supply contributes to the cost of field installations and is also a source of potential failure. Additionally, the need for extra power wires exclude the use of electronic switches as direct replacements for two-wire mechanical devices. There are also maintenance and reliability disadvantages of a battery at the switch.
According to one embodiment of the present invention, there is an interface that provides selected information from a sensor mechanism to a utilization mechanism over a two-wire connection, all power for the interface and the sensor mechanism being received from the utilization mechanism over the two-wire connection. The interface comprises a switch circuit connected between first and second wires of the two-wire connection, the switch circuit providing a high impedance between the first and second wires when open and a low impedance between the first and second wires when closed. The state of the switch circuit is selectively controlled by the sensor mechanism, and current flow through the switch circuit is sensed by the utilization mechanism as indicative of selected information from the sensor mechanism. The interface comprises a voltage regulator that provides a selected voltage to components of the interface and the sensor mechanism, and a current regulator, operative at least when the switch circuit is open, that regulates a current provided to the voltage regulator from the first and second wires to below a predetermined level.
According to another example, an interface provides information from a remote device to a utilization mechanism over a two wire connection, the interface comprising a switch circuit that provides a high impedance between first and second wires of the two wire connection when in an open state and a low impedance between the first and second wires when in a closed state, the state of the switch circuit being indicative of the information from the remote device. The interface also comprises a voltage regulator that provides a regulated voltage to at least one component of the remote device, and a current regulator, coupled to the voltage regulator and the switch circuit, that limits an amount of current provided to the voltage regulator when the switch circuit is in the open state. The interface also comprises a shunt regulator, coupled to the switch circuit, that regulates a terminal voltage across the two-wire connection to a predetermined level when the switch circuit is in a closed state and a bypass that bypasses the current regulator when the switch circuit is in the closed state to enable a higher current to be provided to the voltage regulator. The interface and the remote device receive all operating power from the utilization mechanism over the two wire connection.
The present invention also includes a method of providing selected information from a remote device to a utilization mechanism over a two wire connection, the remote device receiving all operating power from the utilization mechanism. The method comprises providing a signal from the remote device over the two-wire connection and switching a switch circuit of the remote device between a closed state and an open state to provide the selected information from the remote device, the switch circuit passing the signal to the utilization mechanism when in the closed state and the switch circuit presenting a high impedance across the two-wire connection when in the open state. The method also includes providing an input to a voltage regulating device, a regulated output voltage being provided by the voltage regulating device to the remote device, and limiting current when the switch circuit is in the open state to below a predetermined level at which the utilization mechanism interprets the switch circuit as being in the open state. The method further includes regulating voltage across the two-wire connection when the switch circuit is in the closed state to a predetermined value.
According to yet another example, an interface for providing selected information from a sensor mechanism to a utilization mechanism over a two-wire connection comprises a bistable device that provides a high impedance between first and second wires of the two wire connection when in a first state and a low impedance between the first and second wires when in a second state. The interface also comprises voltage regulating means for controlling a voltage provided to at least selected components of the sensor mechanism, and means for limiting current provided to the voltage regulating means from the two-wire connection when the bistable component is in its first state to a current less than that recognized by the utilization mechanism as an indication of the bistable component being in its second state. All power for the interface and the sensor mechanism is received from the utilization mechanism over the two-wire connection.
The foregoing and other objects, features and advantages of the invention will be apparent from the following detailed description of an illustrative embodiment as illustrated in the accompanying drawings.